charts ios port of mpandroidchart

@brief
@property list
ChartData //管理多个数据集，每个数据集可以绘制一条独立的曲线

@property
NSArray<DataSet>

@method
setDrawValues //Enables / disables drawing values (value-text) for all DataSets this data object contains.

@property list
Array<DataEntry> //数据集中的所有数据项

@property list
x //数据项的x坐标
y //数据项的y坐标
icon //数据项显示时的icon

@brief
Each Line-, Bar-, Scatter-, CandleStick- and RadarChart has an XAxis object.

@property
label //数轴旁边的刻度
gridLines //平行于数轴的刻度线

@method
setLabelRotationAngle //设置刻度label的旋转角度

@brief
Each Line-, Bar-, Scatter or CandleStickChart has a left and a right YAxis object, responsible for either the left, or the right axis respectively. The RadarChart has only one YAxis.